1. Field of the Invention
The present invention relates to an existing hot rolling train with a walking beam furnace, a reversing stand following the walking beam furnace and possibly shears and/or descaling devices, as well as a subsequent hot strip finish-rolling train for manufacturing thin, austenitically rolled hot strip.
2. Description of the Related Art
A large number of hot strip rolling trains of the above-described type are in use today. These hot strip rolling trains make it possible to roll hot strip with a thickness of 1.5 mm, or at best 1.2 mm. This is where these hot strip rolling trains reach there limits. A further lowering of the rolled strip thickness is not possible in these trains. There are two reasons for this: on the one hand, the average temperature of the strip at the exit of the last stand of the hot strip finish-rolling train should not drop below about 890.degree. C. to 930.degree. C. in order to ensure an austenitic rolling. On the other hand, the speed at the exit of the last stand of the hot strip finish-rolling train may not exceed about 11.0 to 12.5 m/s, since otherwise the hot strip could no longer be guided without problems on the runout roller table and subsequently reeled. In addition, the threading-in in the hot strip train is problematic in the case of such high speeds.
The slabs are rolled into a preliminary strip in several passes in the roughing train of a hot strip rolling train. A temperature wedge is formed over the length of the preliminary strip which is essentially influenced by the thickness reduction distribution during rolling and the rolling speeds.
In addition, a distribution of the strip temperature over the width of the preliminary strip occurs during the rolling procedure. This results from greater radiation losses at the strip edges and the cooling effect of the descaling water. Consequently, the strip edges may be substantially cooler than the strip center, wherein temperature differences between the strip center and the strip edge in the order of magnitude of 75.degree. C. to 125.degree. C. are possible.
Thus, preliminary strip which is usually introduced into the hot strip finish-rolling train of the hot rolling train has at its edges a temperature level at the lower limits of the temperature necessary of the first roll stand of the hot strip finish-rolling train, but its strip center temperature is still too high. As mentioned above, the strip center temperature at the last stand must be at least about 890.degree. C. to 930.degree. C.
In addition, in the walking beam furnace there are portions of the rolling stock which rest on the beam and are therefore cooled to a greater extent than the remaining portions of the rolling stock, so that the walking beam extending transversely of the rolling direction produces colder areas of the strip extending transversely of the rolling direction of the rolling stock. These so-called skidmarks must also be taken into consideration with respect to the temperature when entering the hot finish-rolling train, wherein the preliminary strip center temperature outside of the skidmarks may still be substantially higher than the strip edge temperature in the areas of the skidmarks.
During rolling in the finishing train, the remainder of the preliminary strip which has not yet been supplied to the rolling process remains on the roller table in front of the finishing table. Consequently, the rearward portion of the preliminary strip remains longer on the roller table when entering the rolling process and is cooled to a greater extent than the beginning of the strip. In order to ensure a constant fixed temperature of the finishing train in spite of this continuously decreasing entering temperature, the rolling speed is continuously increased during rolling. This temperature speed-up compensates for the influences of the decreasing entry temperatures.
In summary, the rolling process in a conventional finishing train is characterized by the following non-constant strip temperatures:
a) non-constant preliminary strip temperature over the strip length;
b) cold areas (skidmarks);
c) non-constant preliminary strip temperatures over the strip width; and
d) greater cooling of the preliminary strip end as compared to the preliminary strip beginning during rolling in the finishing train.